A town in southern China witnessed a once-in-a-lifetime sight when a vibrant rainbow-colored scarf cloud danced above a towering thunderstorm.
The stunning cloud formed above Haikou, a city of two million in far southern China. A billowing cumulus cloud sprouted in the tropical air over Haikou around sunset. It wasn’t the cumulus that drew all the attention—it was the incredible formation that rose above it, called a pileus cloud.
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Pileus, also known as cap clouds or scarf clouds, are slender clouds that develop on top of cumulus or cumulonimbus clouds as they increase.
The moisture condenses exactly along the top of the updraft as a result of the quickly rising air in the towering cumulus pushing against the cooler air above it. The outcome is an elegant pileus adorning the cloud next to it like a cap.
But this wasn’t just any pileus
The highlight of Haikou is the magnificent rainbow tint that the pileus was painted in. Iridescence in clouds is not at all unusual. In the clouds, water droplets and ice crystals cause sunlight to diffract, dividing it into its many wavelengths.
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We rarely see more than a few colors at once. But as the day drew to a close, a fantastic and unusual splash of a vibrant rainbow dancing above the city was caused by the low angle of the sun and the magnitude of the pileus.